Easter isn't just a religious celebration anymore either, yet the church moves it around the calendar far more than St George's day and everyone seems to follow it. They invented it so their rules apply.

Fair point, though the interweb seems to suggest that nobody is really paying attention to the moving of St George's Day. Not that you could really tell, seeing as nobody pays attention to it full stop.

Most people adhere to the Church's Easter move, because there's a vast international army of Catholic zombies following them.